C/C++编程：pair

青旅半醒 2022-05-19 11:40 160阅读 0赞

头文件

#include <utility>

#  #

1、pair是一个struct，而不是一个class，因此所有的成员都是public：

template<class _T1, class _T2>
        struct pair
        {
          typedef _T1 first_type;    /// @c first_type is the first bound type
          typedef _T2 second_type;   /// @c second_type is the second bound type
    
          _T1 first;                 /// @c first is a copy of the first object
          _T2 second;                /// @c second is a copy of the second object

2、初始化

std::pair<int, float> p;                           // 默认调用int(), float()初始化p中的元素
     std::pair<int, const char*> p1(42, "hello world");
    
    
     std::pair<int, char>(42, '@');
     make_pair(42, '@');                               // 等效于 std::pair<int, char>(42, '@');

3、拷贝构造函数同时存在两个版本：

*  接受相同类型的pair
 *  隐式转换的pair

#include <utility>
    #include <string>
    void f(std::pair<int, const char*>){};
    void g(std::pair<const int, std::string>){};
    int main()
    {
        std::pair<int, const char*> p(42, "hello world");
    
        f(p);  // OK: calls implicittly generated copy
        g(p);  // OK: calls template constructor
    
        f(std::make_pair(42, "hello world"));
        g(std::make_pair(42, "hello world"));
        
        f({42, "hello world"});
        g({42, "hello world"});
        
    }

4、tie：抽取出pair中的value

#include <utility>
    #include <string>
    #include <functional>
    
    void f(std::pair<int, const char*>){};
    void g(std::pair<const int, std::string>){};
    int main()
    {
        int i = 0;
        auto p = std::make_pair('x', 'y');
    
        char c;
        std::tie(std::ignore, c) = p;   //ignore one, ref two
        printf("%c\n", c);  // y
    
    }

5、元祖与元祖中的元素

#include <iostream>
    #include <utility>
    #include <tuple>
    using namespace std;
    
    class Foo {
    public:
        Foo (tuple<int, float>) {
            cout << "Foo::Foo(tuple)" << endl;
        }
        template <typename... Args>
        Foo (Args... args) {
            cout << "Foo::Foo(args...)" << endl;
        }
    };
    
    int main()
    {
        // create tuple t:
        tuple<int,float> t(1,2.22);
    
        // 将元组作为一个整体传递给Foo的构造函数:
        pair<int,Foo> p1 (42, t);
    
        // 将元组的元素传递给Foo的构造函数:
        pair<int,Foo> p2 (piecewise_construct, make_tuple(42), t);
    }

6、pair中move与ref

#include <utility>
    #include <string>
    #include <functional>
    
    
    int main()
    {
        int i = 0;
        auto p = std::make_pair(std::ref(i), std::ref(i));  // 引用类型
        ++p.first;
        ++p.second;
    
        printf("i = %d\n", i);
    
    
        int i1 = 0;
        auto p1 = std::make_pair(std::move(i1), std::move(i1));
        ++p1.first;
        ++p1.second;
    
        printf("i1 = %d\n", i1);
    }

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[20210207160948865.png]: /images/20220519/af1add4af8234722989fd2a6657c16b9.png